Light Transmissivity of Tree Shelters Interacts with Site Environment and Species Ecophysiology to Determine Outplanting Performance in Mediterranean Climates

Plastic tree shelters are commonly used in plantations under Mediterranean climates to protect against herbivory and enhance outplanting performance. However, effects on outplanting performance cannot be generalized due to the complexity of plant responses to microenvironmental conditions within the tube wall. The interactions between the light transmissivity of the tubes and species-specific responses to light and site environment on two-year outplanting performance were studied in two species with contrasting shade tolerance planted inside tree shelters with four different light transmissivities and a non-tree shelter control at two Mediterranean sites with contrasting rainfall and temperature. In general, increasing light transmissivity enhanced biomass accumulation, suggesting that the use of clear tubes might be advisable. However, the shade-tolerant Q. ilex did not benefit from the greater light transmissivity in the most arid site, indicating that the positive effect of clear tubes depends on water stress experienced by seedlings, which ultimately is determined by drought resistance strategies and site conditions. The growth of both species and survival of P. halepensis were higher within clear tubes in the continental site than in unsheltered plants, which suggests that factors other than light, such as warmer daytime temperatures or the prevention of dust deposition, can explain this beneficial site-dependent effect of tree shelters. In conclusion, our results confirm the hypothesis that the effect of tree shelter and its light transmission on outplanting performance is site and species-specific, but further research is needed to identify the effect of other effects not related to light transmission.

Assessing the Efficacy of Seedling Planting as a Forest Restoration Technique in Temperate Hardwood Forests Impacted by Invasive Species

The emerald ash borer (Agrilus planipennis Fairmaire; EAB) is an invasive insect that causes mortality of trees in the genus Fraxinus, creating canopy gaps that may facilitate invasion by exotic plants. Planting native tree seedlings under EAB-infested Fraxinus may accelerate succession and preclude invasive plant expansion; however, the effectiveness of this approach has not been experimentally tested. We assessed understory seedling planting of Quercus rubra, Carya laciniosa, and Juglans cinerea in EAB-infested forests, where the invasive shrub Lonicera maackii (Amur honeysuckle) was removed. We tested whether the use of plastic tree shelters (“tree tubes”) or planting season (fall versus spring) contributed to the success of the reforestation plan by measuring growth rates (cm/yr) and survivorship two and seven years after planting. After seven years, seedling survivorship was <25% for all species and planting techniques. Quercus rubra exhibited poor survivorship with one seedling surviving to the conclusion of the experiment. Juglans cinerea and C. laciniosa had higher survivability and growth rates than did Q. rubra after two and seven years. Effects of tree tubes were weak and temporary. After 2 years, Q. rubra seedling survivorship was higher in tree tubes; however, by the end of the experiment 29 of the 30 Q. rubra seedlings in tree tubes had died. Juglans cinerea seedlings grew faster when planted in the fall compared to the spring, but overall survivorship of these seedlings was unaffected by planting season. Neither the use of tree shelters nor the planting season contributed to the growth or survival of C. laciniosa seedlings. In summary, our results indicate that seedling planting of Carya and Juglans may be a useful way to increase biodiversity in regenerating forests; however, the resource-expensive processes of over-wintering seedlings and using tree shelters may not increase the success of reforestation efforts.

The Effect of Polypropylene Tree Shelters on Growth and Survival of Pedunculate Oak Seedlings (Quercus robur L.)

Background and Purpose: The declining and dieback of lowland oak forests as the result of global climate change, as well as the attack of various pathogenic organisms, industrial pollution, and other negative effects reduce acorn yield and will continue to do so in the future. As a result of this fact, the areas on which artificial regeneration will be applied by planting seedlings will have to be increased. The artificial regeneration process is more expensive than the natural one, so protection measures need to be applied to minimize the loss of young seedlings. Materials and Methods: Experimental plots were set up in the forest area of Spačva Basin. A completely randomized block design experiment in four repetitions was used, in which four variants of planting and protection of oak seedlings were examined. We tested polypropylene tree shelters and hydrophilic superabsorbent based on maize starch. Heights of the plants were measured by measuring tape, and survival was registered by numbering of dead and living plants. For statistical purposes, we have used analysis of variance (repeated measure ANOVA), which allows us to prove statistically significant difference between morphological traits of studied variants. Results: Two years after planting, the lowest level of survival was found in unprotected seedlings (79.5%), while tree shelters provided the best conditions for seedlings survival (88.75%). In the case with the hydrophilic granules Zeba the result is slightly worse. When analyzing average height in a particular case, significantly higher plants are recorded in cases where the polypropylene tree shelters are used. The average height, with tree shelters (128.27 cm), is more than three times higher than the unprotected seedlings (37.97 cm). The addition of hydrophilic granules did not show a statistically significant difference. In cases where the seedlings were not under the tree shelter protection, the effect of hydrophilic granules was also not statistically significant. Conclusions: Sheltering accelerated the height growth of the planted oaks substantially and also provided best conditions for seedlings survival. During the research period the usage of Zeba did not justify the purpose for which it was applied. Although the first results are promising in order to gain a complete insight into the potential of tree shelters it is necessary to record the changes and maintain the experiment for a further 2-3 years with quantification of other harmful factors that may appear (drought, game etc.) after which a more reliable judgment regarding their usefulness could be provided.

Ecological Factors Affecting White Pine, Red Oak, Bitternut Hickory and Black Walnut Underplanting Success in a Northern Temperate Post-Agricultural Forest

This study took place in southern Québec (Canada) where young stands of white ash and grey birch have been underplanted with white pine, red oak, bitternut hickory and black walnut. The establishment success of white pine and red oak was measured with and without tree shelters (to protect from deer). Ecological factors affecting the height growth of the four species were also measured for protected trees. After 6 years, the survival and total height of unprotected oak was 29% and 44.3 cm vs. 80.5% and 138.5 cm for protected oak. White pine was less affected by browsing (survival of 79.5 and 93.5%; height of 138.5 and 217.9 cm for unprotected vs. protected pine). Height of white pine was higher in the grey birch stands, while height of all hardwoods was higher in the white ash stands, which had better soil drainage, higher fertility, and an understory dominated by Rubus species. Total height of all hardwoods was significantly (p < 0.05) correlated with Rubus cover and with soil fertility. Pine and walnut height were strongly correlated (p < 0.001) to shelterwood structure (canopy openness or total basal area). Pine was less sensitive to variations in shelterwood characteristics, while black walnut showed high sensitivity. This study provides evidence that underplanting is suitable for black walnut assisted migration northward and for bitternut hickory restoration, despite soil conditions that were less favorable than in bottomland habitats mainly supporting these species in eastern Canada. Tree shelters offering protection from deer browsing and species-specific site selection are recommended for underplanting in the southern Québec region.